Solid state image sensors are classified into three groups, i.e. MOST (Insulated-Gate Field Effect Transistor) type image sensor, CTD (Charge Transfer Device) type image sensor, and CID (Charge Injection Device) type image sensor, each of these types having a specific photo-electric conversion function and scanning function required for use in image pickup devices (see Nagahara et al "MOS Type Area-Imaging Sensor and Television Camera", the Japanese journal of Electronics, April 1976, p.p. 368-372).
An MOST type solid-state image sensor, as described in the above-referenced Nagahara et al publication, will be described as an example with reference to FIG. 1.
In FIG. 1 schematically showing an MOST type solid-state image sensor, reference numeral 11 designates a horizontal scanning circuit for X-position or column addressing, numeral 12 a vertical scanning circuit for Y-position or row addressing, numeral 13 vertical switching insulated-gate field effect transistors (hereinafter referred to simply as MOSTs) which are turned on and off by vertical scanning pulses from the vertical scanning circuit 12, numeral 14 photodiodes formed by using the source junctions of the MOSTs 13, numeral 15 a vertical signal output line connecting the drains of the MOSTs 13 of the same column in common, numeral 16 horizontal scanning MOSTs which are turned on and off by horizontal scanning pulses from the horizontal scanning circuit 11, with their drains connected with a horizontal signal output line 17 and their sources connected with the vertical signal output line 15, numeral 18 a driving voltage source (i.e. voltage source for video output) connected via a resistor 19 with the horizontal signal output line 17 to drive the photodiodes 14, and numeral 20 a signal output terminal. The horizontal and vertical scanning circuits 11 and 12 respectively regulate the horizontal and vertical switching MOSTs 16 and 13 one by one and the photocurrents from the photo-diodes 14 arranged in a two-dimensional array are read out through the resistor 19. Since the respective signals from the photodiodes 14 correspond to an optical image projected thereon, the currents read out of the photodiodes provide the original video signal.
The above-described solid-state image sensor is featured in that it can use the sources of the switching MOSTs for its photo-electric conversion functions and that MOST type shift registers can be used for the scanning circuits. Accordingly, this type of solid-state image sensor can be easily fabricated with high integration and therefore realized by using MOS LSI technology, as shown in FIGS. 2A and 2B. FIG. 2A shows in cross section the structure for one picture element and FIG. 2B shows in plan view a part of the picture element arrangement. In these figures, reference numeral 23 designates a semiconductor substrate of N-type conductivity on which photo-electric conversion elements and scanning circuits are integrated, numeral 24 a well region or impurity region of P-type conductivity formed on the N-type semiconductor substrate 23, numeral 13 a vertical switching MOST having a gate electrode 25 to which the vertical scanning pulse from the vertical scanning circuit 12 is applied, numeral 26 a high impurity concentration region of N-type conductivity which serves as the source of the MOST 13 and also forms a photodiode 14 together with the P-type well region 24 with a PN junction formed therebetween, numeral 27 a high impurity concentration region of N-type conductivity which serves as the drain of the MOST 13 and is connected with one end of a conductor layer 28 as the vertical signal output line 15. The other end of the output line 28 or 15 connected with the drains of plural such vertical switching MOSTs 13 in common is connected with one end of a horizontal switching MOST 16 which is turned on and off by the horizontal scanning pulses from the horizontal scanning circuit 11 and the other end of the MOST 16 is connected with the horizontal signal output line 17. The well region 24 and the substrate 23 are usually kept at earth potential (OV). (Sometimes, the PN junction between the well region 24 and the substrate 23 may be reversely biased.) Reference numerals 291, 292 and 293 designate insulating films which usually comprise SiO.sub.2.
In operation, the photodiode 14 is charged up to the video voltage V.sub.v at one scanning but it discharges by .DELTA.V.sub.v in accordance with the quantity of light incident thereon during the period of one field. When the associated switching MOSTs 13 and 16 are turned on at the next scanning, this discharging function is compensated by a charging current from the video voltage source 18. This charging current corresponding to the discharging function is read out through the resistor 19 connected with the video voltage source 18 so that a video signal is obtained from the output terminal 20.
Since the solid-state image sensor having a picture element structure shown in FIGS. 2A and 2B, as disclosed in U.S. Pat. No. 4,148,048 patented Apr. 3, 1979, has the P-type well region in which the photo-electric conversion element is formed, the sensor is free from blooming. Moreover, with this sensor, infrared light is almost completely absorbed in the substrate so that the resolution is prevented from being degraded, and the spectral response in the visible range has a flat or balanced characteristic so that the video signal of an object can be obtained with high fidelity. This device may be said to have greatly improved properties over any other image sensors hithertobefore reported and developed.
The other MOST type image sensor, the CTD type image sensor or the CID type image sensor may be used for a solid-state color camera, as well.
FIG. 3 schematically shows a three-chip type color camera using three solid-state image sensors. Referring to FIG. 3, light having passed through a lens 31 is decomposed into red (R), green (G) and blue (B) components by, for example, a dichroic prism 32 for color separation. The R, G and B components are focused respectively on solid-state image sensors (hereinafter referred to also as an imager) 34, 33 and 35 for the components R, G and B. The R, G and B imagers 34, 33 and 35 perform photo-electric transducing functions. In a conventional solid-state color imaging camera, the optical positioning of the imagers 33, 34 and 35 for each picture element has an exactly overlapped registeration for preventing color breakup. The resolution of the color imaging camera is equivalent to that of each of the imagers 33, 34 and 35 for white light.
In television (TV) broadcastings, one frame contains 525 horizontal scanning lines in the case of NTSC format employed in USA and Japan. The number of picture elements in the vertical direction for each imager is required to be the same as that of the horizontal scanning lines, or at least about 500 elements if some for vertical (or blanking) intervals may be omitted. As for the number of picture elements in the horizontal direction, it must be at least about 400 to obtain a content picture quality though depending upon a desired resolution. As a result, each of the imagers 33, 34 and 35 would have a large chip size corresponding to that of a memory LSI having an extremely large scale of more than 200 K bits. Further, according to the NTSC format, the ratio of the picture size in the vertical direction to that in the horizontal direction is as small as 3/4, and therefore it is necessary to place a greater number of picture elements in this smaller (vertical) direction. Such an arrangement, however, is very difficult. Therefore, the size of the imager would be extraordinarily large as compared with common semiconductor LSI devices, thereby greatly decreasing the production yield and rendering the fabrication difficult. In addition, an optical system such as the dichroic prism must be correspondingly large. These lead to the lack of the predestined merits as a solid-state imaging camera or color TV camera with compactness, small weight and inexpensiveness and also prohibits the realization thereof.
The TV scanning format is an interlaced scanning in which every other horizontal scanning line is traced during one field and the remaining horizontal lines are successively traced during the next field to complete one frame. The photo-electrically converted signals derived from the imagers 33, 34 and 35 must be adapted for this scanning format. For explanation of this interlaced scanning, an imager 41 simplified to have a 6.times.6 picture element matrix is shown in FIG. 4.
Referring to FIG. 4, of all the horizontal rows A.sub.1, B.sub.1, A.sub.2, B.sub.2, A.sub.3 and B.sub.3 of picture elements 42, the rows A.sub.1, A.sub.2 and A.sub.3 are scanned in the odd-numbered fields while the rows B.sub.1, B.sub.2 and B.sub.3 are scanned in the even-numbered fields, according to the above-described interlaced scanning.
To achieve such a scanning, it is necessary to provide a complicated switching mechanism which performs the switching of the scanning lines for every field in the imager 41 (33, 34 and 35 in the case of the system shown in FIG. 3).
Further, according to the above scanning, signals remaining in the skipped rows B.sub.1 .about.B.sub.3 after the completion of the scanning of the rows A.sub.1, A.sub.2 and A.sub.3 in the odd-numbered field provide a superposition effect on signals read out of the rows B.sub.1, B.sub.2 and B.sub.3 at the next scanning in the even-numbered field, thereby generating an undesirable after-image or lag on the reproduced picture. To eliminate such an after-image, it is necessary to read out the signals from all the picture elements in every field. For this purpose, pairs of rows A.sub.1 and B.sub.1, A.sub.2 and B.sub.2, and A.sub.3 and B.sub.3 are successively scanned in the odd-numbered fields and different pairs of rows, for example, B.sub.1 and A.sub.2, B.sub.2 and A.sub.3 etc. are successively scanned in the even-numbered fields. Consequently, the afore-mentioned switching mechanism would be further complicated and also complicated treatment of signals would be required. Moreover, in the case of the NTSC format, the number of picture elements in the vertical direction must be as large as about 500 to cover one frame, as mentioned above.